Project II: Summary/Abstract Gait dysfunction and postural instability are common in Parkinson disease (PD), and a significant cause of treatment-refractory disability. Accumulating evidence implicates cholinergic dysfunction as a significant contributor to gait and balance impairment, even in early disease. During the initial funding period we established the vesicular acetylcholine transporter (VAChT) ligand [18F]FEOBV, which uniquely allows assessments of high cholinergic binding areas, such as the striatum. Our preliminary PET studies suggest that both isolated falls and FoG status are associated with VAChT reductions in the striatum. Combined with preclinical data from Project I, our findings suggest a previously unappreciated role for striatal cholinergic interneurons as a common denominator of attentional-motor integration in the etiology of falls and FoG. Our preliminary data show that distinct patterns of extra-striatal cholinergic changes, combined with the striatal abnormalities, associate with distinct clinical features. PD subjects with isolated falls show additional changes in the visual thalamus and prefrontal cortex, whereas those with FoG exhibit thalamic and limbic (i.e., hippocampus, amygdala and anterior cingulum) abnormalities. These findings implicate cholinergic changes within striatal, cortical, thalamic and limbic circuitry subserving motor-attentional and sensory-attentional network dysfunctions in PD fallers and freezers. We hypothesize that dissecting the temporal nature of these extended cholinergic network dysfunctions may provide systems-level insight into the progressive pathophysiology from falling to FoG in PD. Within the striatum, cholinergic changes coexist with dopaminergic (DA) denervation, and our model (Overall Component) outlines how these abnormalities synergize. DA denervation occurs in additional CNS regions, where it may also synergize with cholinergic abnormalities. The assessment of extra-striatal DA nerve terminals requires highly selective ligands. To assess our hypotheses about the impact of cholinergic and dopaminergic changes on PD gait dysfunction, we propose to perform a prospective cohort study with [18F]FEOBV and the high affinity dopamine transporter (DAT) [11C]PE2I brain PET at baseline and 2-year follow-up in PD subjects at risk of conversion to isolated falls versus falls complicated with FoG. Novel insights in cholinergic and extra-striatal dopaminergic changes underlying falls and FoG may inform new therapeutic interventions to treat these debilitating motor complications. Project II is highly integrated thematically with Project I, complementary to Project III, and will interact extensively with all Cores. Our work also will be leveraged by a unique, deeply phenotyped cohort of PD participants developed in the prior funding cycle allowing us to recruit an enriched sample of patients likely to convert to fall and FoG status to allow unique longitudinal within-subject assessments.